Manufacturing a semiconductor device or a liquid crystal display device is performed in a highly clean space. To create a highly clean space, a porous membrane composed of polytetrafluoroethylene (PTFE) (hereinbelow, also referred to as a “porous PTFE membrane”) is used as a filter for collecting fine particles. From the viewpoint of having high efficiency for collecting fine particles compared to a filtering medium composed of glass fiber with the same pressure loss, the porous PTFE membrane is particularly preferably used for a HEPA filter (High Efficiency Particulate Air Filter) or a ULPA filter (Ultra low Penetration Air Filter).
As a HEPA filter, a filtering medium composed of glass fiber has been conventionally used. However, there is a problem that it has a high pressure loss. Meanwhile, a filter with a low pressure, which is achieved by applying the porous PTFE membrane to a filtering medium, has been also conventionally commercialized. However, there is a problem that, since a pressure loss easily increases due to an occurrence of surface filtration as caused by a dense structure of the porous PTFE membrane, the life span is equal to or less than half of a glass fiber filter.
In order to increase the life span, a filtering medium adhered with an MB (Melt Blown) non-woven fabric at upstream side of the porous PTFE membrane is also suggested. However, because the filtering medium thickness increases according to adhesion of MB non-woven fabric, and thus there is a problem that an area to which the folded filtering medium is inserted while it is incorporated into an air filter unit is reduced and also the price of the filtering medium increases.
With regard to a porous PTFE membrane of a related art, it is known that high air permeability is obtained by adding PTFE that is not fibrillated by drawing to PTFE that is fibrillated by drawing during manufacture (for example, see Japanese Patent Publication No. 10-505378 W and 2814574 B1). This porous PTFE membrane has fibrils and knotted portions that are connected to each other by fibrils. However, since the strength of the knotted portions is weak, the membrane structure changes as compressive force or the like is applied during a post step like laminating an air permeable support, and thus the filter performance such as a pressure loss, PF value, or filter life span may be deteriorated.